I would consider servicing the tranny (Replace Filter & Fluid) before adding something to clean it out. But you would want to look at replacing the solenoid, seems to be a common fix during this sort of problem. Not a guaranteed fix every time but several cases have been the solenoid gone out or going out.

I am pretty sure the cost to replace the TCS is around $800. That'* why I am looking for a work-around by installing the shift kit.